FIG. 1 illustrates one representative prior art linear voltage regulator 10, which is an LT3080 low dropout (LDO) regulator manufactured by Linear Technology Corp. An LDO regulator is generally synonymous with a linear voltage regulator, and the term “low dropout” refers to the small minimum voltage differential that can occur between the input voltage terminal and the regulated output voltage terminal while still achieving regulation.
LDO regulators operate by varying the conductivity of a pass (or series) transistor, connected between the input terminal and output terminal, to achieve a predetermined output voltage. The output level of an operational amplifier (op amp), which is a type of differential amplifier, controls the conductivity of the pass transistor. Typically, the regulator's output voltage is fed back into one input terminal of the op amp, and the conductivity of the pass transistor is controlled to match the output voltage to a reference voltage applied to the other input of the op amp. The user selects the reference voltage. Alternatively, a divided output voltage is fed back and matched to a fixed reference voltage, where the user selects resistors for the divider to achieve the desired output voltage.
In FIG. 1, the power supply voltage may be applied to both the Vin pin 12 and the Vcontrol pin 14 of the IC package. The user connects an Rset resistor 15 between a Set pin 16 and system ground to set the output voltage Vout provided at the Vout pin 18. A fixed precision current source 20 supplies a fixed current through the Rset resistor 15 to generate a reference voltage Vref at the non-inverting input of the op amp 22. The output voltage Vout is applied to the inverting input of the op amp 22. The terms inverting and non-inverting simply refer to the two branches of the differential amplifier in the op amp 22, shown in FIG. 2. It is assumed that the op amp 22 includes a driver supplying the required current for driving the base of the pass transistor 24.
Using an internal current source 20 and Rset resistor 15 to set the reference voltage is preferred to dividing the output voltage and matching the divided voltage to a fixed bandgap reference voltage source (typically about 1.2 volts), since, by using the current source, the loop gain and bandwidth of the regulator are not affected by the output voltage, as the regulator will always be in a unity gain configuration.
The op amp 22 controls the conductivity of the pass transistor 24 so that Vout matches Vref. Such an op amp in a regulator application is also referred to as an error amplifier.
In one application of the regulator 10, the op amp 22 has its power terminals connected to the Vout pin 18 and to the power supply voltage Vin, by externally shorting the Vcontrol pin 14 to the Vin pin 12. This allows the regulator to be “floating” and used in high voltage applications.
In a typical example, the op amp 22 needs about 1.4 volts across its power terminals in order to operate properly. Accordingly, when the Vcontrol pin 14 is tied to the Vin pin 12, the LDO regulator 10 (a positive voltage regulator) can only regulate Vout to within 1.4 volts of Vin in order for the op amp 22 to be adequately powered. It would be desirable for various reasons, including efficiency and battery life, to enable regulation within 1.4 volts of Vin. Accordingly, for non-high voltage applications, the data sheet for the LT3080 (FIG. 1) describes the option of connecting the Vcontrol pin 14 to a voltage higher than Vin so that the differential between Vin and Vout can go down to the Vce saturation voltage of the pass transistor 24 (typically 100-500 mV, depending on the load current), while still providing at least 1.4 volts to power the op amp 22. However, most applications do not already have a separate voltage source higher than Vin.
The LT3080 does not allow any power terminal of the op amp 22 to be tied to system ground. Being able to ground the op amp 22 provides various advantages, including low dropout voltage and independence from Vout.
Other types of voltage reference-based regulators always require the ground pin to be tied to the system ground, so there is no versatility in the ground pin coupling.
What is needed is an LDO regulator IC that has more versatility in the connection of its ground pin. This would allow the regulator's ground pin to be connected in a way that is most optimal for the particular application, such as low input voltage, high input voltage, low output voltage, regulation close to Vin, regulation close to ground, etc. Further, the concept should be applicable to both positive voltage regulators and negative voltage regulators.